Well here we are on the Stored Product Insect page...these are, in most cases, very small insects which infest products like grain, flour, cheese,chocolate etc..etc..etc,(and I don't just mean in packets..I'm talking in thousands of tons). People who are not in the industry of food production or pest control never really realise what a battle is fought to make sure that the products which come to you are just the product with no added protein (ha-ha). Food factories, bakeries, supermarkets, suppliers, farmers, breeders, the list goes on and on, all are bound by the Food Regulations and Environmental Health to ensure that the environment they produce their product in is the cleanest and most hygienic possible. One of the main aspects of this regime is pest control.

So many of the products used in this country are of course imported, hence there are very strict regulations governing the risk of also importing pests along with the product. In the case of containers these are fumigated with gas (depending upon the goods).

Fumigation: Technical aspects

Let us get one thing quite clear for those who do not work in the pest control or food industries: Fumigation is the use of a gas (not the spraying of insecticides) to destroy pests which may infest a building or a product, an item of furniture, a car in fact virtually anything can be fumigated, there are exceptions but generally this is the case. Let us look at a quick example:

Say a customer rings up and says that they suspect that they have woodworm in a beautiful regency sideboard, can we do anything to help, will it damage the item, etc, etc. Well yes we can help and we go along and pick up the item, take it to our gas chamber, or conversely we can seal it into what is called a "stack", which is parcelled up in heavy gauge polythene. Once we are sure that everything is gas tight we can calculate the volume of the stack or chamber and thereby calculate the dose of gas required (there are different doses for different pests at varying temperatures) and introduce it, this is a skilled job and can only be carried out by highly qualified personnel.

There are various gases which can be used these are:

(1) Methyl Bromide (CH3Br) supposedly to be phased out by 2005 (possible ozone depleter).
(2) Nitrogen (N2)
(3) Phosphine (PH3)
(4) Carbon Dioxide (CO2)

OK..! back to our example. The item is left under gas for a certain time period, this varies with which gas is used, in the case of methyl bromide used on woodworm it can be 48 - 72 hours, if it was done with carbon dioxide it can be 2 or 3 weeks. Anyway, once the required time period has elapsed, the qualified personnel then return and vent off the chamber or stack, that is let the gas out, and using various instruments, determine when the item is free of the gas and able to be returned to the customer.

The thing about fumigation is that it isn't residual...it will kill the pests in the item and it will also kill the eggs, which spraying doesn't, but it does not leave a barrier to protect against re-infestation.

So in the case of a grain store of food factory etc, if there were a need to use fumigation, it should be used in conjunction with a spraying program and the situation should be closely monitored. I'm all for using pesticides if it is needed, but only in the absolute necessary amounts, and not bucket-fulls, this is why monitoring is needed so that only the areas which need treatment get it.

Technical Aspects of Fumigation:

Fumigants are low molecular weight chemicals, highly toxic and volatile, that are used during storage to kill all insect stages residing in the produce. Fumigation is a widely used method all over the world on small as well as large storage scale. The method can be applied at the farm level in gas-tight granaries or silos, under gas-tight sheets carefully covering the product or at a large scale storage as in large warehouses.

Fumigants are commercially available in a solid, liquid or gaseous state. Phosphine (PH3), for example, is a formulated fumigant commercially available as either tablets, pellets, bags or plates. Methyl bromide (CH3Br), on the other hand, is gaseous in form and packed in a liquid form in pressurised steel bottles. At temperature above 4C it takes a gaseous state, thus, once the container is opened, the gas is released and starts to act as a fumigant. The two compounds are the most widely spread fumigants in use.

However, a problem of human toxicity due to inadequate application of the method is considered a drawback regarding this industry, specially in the developing countries, where inappropriate handling of such toxicants is widespread. Another problem with the use of fumigants has recently aroused, which is the developing of resistance from insects against fumigants. The problem started as a result of improper application of the chemicals in use, i.e. application of incorrect doses, fumigation in non gas-tight containers or insufficient exposure time. Recently, fumigation has been highly discouraged at a small-scale level. moreover, the use of methyl bromide has been strongly restricted in industrialised countries because of its ozone-depleting potential. However, fumigation is still the most widely operated method as an essential large scale post-harvest practice.

The search for other alternatives

Trials have been conducted on the use of carbon dioxide as a fumigant to replace methyl bromide in the control of insects and mites damaging stored products. The use of CO2 rich atmospheres showed promising results in disinfesting food commodities in small storage facilities. A relatively new technique used by the Indonesian National Logistic Agency (Bulog) for milled rice is to seal bag sticks into large plastic enclosures flushed with carbon dioxide (Hodges & Surendro, 1996).

Treatment with high-pressure carbon dioxide under different temperatures may result in different rates of mortality, for example, at 15°C, 95 percentage mortality of Lasioderma serricorne was observed after 38.5 min of treatment, while the same level of control was achieved within 1 minute at 45°C (Ulrichs, 1995). Corinth & Rau (1990) showed that each tonne of grain requires about 19-27 kg carbon dioxide to achieve complete mortality of Oryzaephilus surinamensis, Tribolium castaneum and Sitophilus granarius in 4-6 weeks.

The use of "Biogas" as a fumigant, with methane and carbon dioxide as its main components, may achieve good results in the control of stored pests. Subramanya et al.(1994) showed that biogas significantly reduced infestations and loss in stored pigeon pea infested with Callosobruchus chinensis. Gursharan et al. (1994) recorded up to 100 percentage mortality of Sitophilus oryzae, Rhyzopertha dominica, Trogoderma granarium and Tribolium castaneum after six days' exposure to biogas in PVC bins. Another method for the control of insects in industrial premises was developed, where a Gas Operated Liquid Dispensing system was used to mix separate sources of carbon dioxide and insecticide concentrate. The system, given the name Turbocide GOLD, produces a fine insecticidal aerosol that was reported to give excellent control of Tribolium castaneum, T. confusum and Lasioderma serricorne (Groome et al., 1994).

Several studies have focused on developing post-harvest technologies as a key role in ensuring food security. Consumers are now aware of the danger in the use of chemical pesticides to protect stored products. This, and the world-wide trend to minimise the use of toxic substances applied on food products, have led scientists to seek less dangerous alternatives. Fumigation, for example, has become an endangered technology due to pressures regarding environmental contamination and health concerns.


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© Stuart M Bennett 2003